Abstract : Isoplethic thermal analysis was used to determine the solid-liquid-liquid equilibria in the ternary system water-sodium sulfate-piperidine. The changes in state observed on the thermogram recorded during the displacement in a quasi-binary section permit the identification of the different phases and the delimitation of the corresponding equilibrium domains. Two isotherms were established at 25°C and 40°C because these temperatures frame the peritectic decomposition of the sodium sulfate decahydrate. Miscibility gaps were completely delimited and each critical point was calculated. This study permitted to determine the optimal conditions for separating the organic phase by liquid-liquid extraction.